Job 38 (where God challenges Job):
Who is this that obscures my plans with words without knowledge? Brace yourself like a man; I will question you, and you shall answer me. Where were you when I laid the earth's foundations? Tell me, if you understand. Who marked off its dimensions? Surely you know! Who stretched a measuring line across it? On what were its footings set, or who laid its cornerstone while the morning stars sang together and all the angels shouted for joy? Who shut up the sea behind doors when it burst forth from the womb, when I made the clouds its garment and wrapped it in thick darkness, when I fixed limits for it and set its doors and bars in place, when I said, "This far you may come and no farther; here is where your proud waves"?
Have you ever given orders to the morning, or shown the dawn its place, that it might take the earth by the edges and shake the wicked out of it? The earth takes shape like clay under a seal; its features stand out like those of a garment. The wicked are denied their light, and their upraised arm is broken.
Have you journeyed to the springs of the sea or walked in the recesses of the deep? Have the gates of death been shown to you? Have you seen the gates of the deepest darkness? Have you comprehended the vast expanses of the earth? Tell me, if you know all this.
What is the way to the abode of light? And where does darkness reside? Can you take them to their places? Do you know the paths to their dwellings? Surely you know, for you were already born! You have lived so many years!
Have you entered the storehouses of the snow or seen the storehouses of the hail, which I reserve for times of trouble, for days of war and battle? What is the way to the place where the lightning is dispersed, or the place where the east winds are scattered over the earth? Who cuts a channel for the torrents of rain, and a path for the thunderstorm, to water a land where no one lives, an uninhabited desert, to satisfy a desolate wasteland and make it sprout with grass? Does the rain have a father? Who fathers the drops of dew? From whose womb comes the ice? Who gives birth to the frost from the heavens when the waters become hard as stone, when the surface of the deep is frozen?
Can you bind the chains of the Pleiades? Can you loosen Orion's belt? Can you bring forth the constellations in their seasons or lead out the Bear with its cubs? Do you know the laws of the heavens? Can you set up God's dominion over the earth?
God makes it painfully clear: When we start to speculate about the mind of God and what happened during creation (beyond what He has revealed to us) we tread on very shaky ground.
Who knows the unfathomable mind of God? For no one knows the thoughts of God except the Spirit of God (1 Cor 2:11). Who has known the mind of the Lord so as to instruct him? (1 Cor 2:16)
This is why I can't support Ussher's 6000 year calendar that starts at 4004 BC. It is not because it is repeatedly and gleefully used as a cudgel by atheists to beat believers sensesless rhetorically (which is why you see it brought up in the media all the time).
The problem is that a 6000 year timeline is not Biblical. It is based mainly on a misreading of 2 Peter 3:8 (which is quoting Ps 90:4), where Peter states with the Lord a day is like a thousand years, and a thousand years are like a day.
Ussher did a basic misreading of Peter. What he did was simple. God created the world in 6 days, a day to God is a 1000 years, so 6 times 1000 years = 6000 years, QED. But don't you see? That's not what Peter wrote! He wrote a thousand years is like a day to Him, not that it literally was.
I am not arguing that it is merely a metaphor. Why? Because I don't have to. 2 Peter 3:8 is not a metaphor, it is something much stronger. It is a simile ('like a'). Peter wrote 'like a' very deliberately, I think, to prevent exactly the kind of bogus literal misinterpretation that Ussher did. To underscore that fact Peter intentionally used two similes and reversed them: that from God's perspective it can go either way. In other words, I think the point Peter is making is actually quite clear: that the passage of time is basically irrelevant from God's perspective.
This is why I make a silent sigh whenever I see a Young Earth creationist argue the 6000 year timeline with an atheist. Invariably it only serves to reinforce the atheist's unbelief. Why? Not because a I think a belief in a 6000 year-old Earth is scientifically wrong, but because the Bible does not demand it. I will admit that the Earth really might be 6000 years old - I don't know because I wasn't there - but we cannot demand it, and in particular we should not base our witnessing on it to unbelievers as a condition of their salvation.
This is why I think hanging your hat on Mark 10:6 as an assertion that Adam and Eve were created immediately after the stars and the heavens is very dicey. What does 'From the beginning' actually mean to a God that exists outside of time and space? Where you there? Do you know the mind of God? Yes, Adam and Eve were created as the first man and woman, and yes they sinned and Adam fell. But beyond what scripture reveals to us you cannot safely go.
Basically, don't try to put God in a box. He wants us to marvel as His creation, for the heavens declare the glory of God; the skies proclaim the work of His hands (Ps 19:1). Just give glory to God and praise Him for the majesty of His creation, of the universe He created with 100 billion galaxies each with 100 billion stars, and revel in coolness of his Plan for us and what He wrought, and be thankful for the underserved gift of salvation that we receive in the blood of Christ Jesus.
Usher did an excellent job.
And no, we should not make belief in a young earth a condition for salvation.
However, this is really not the central issue of the young earth/old earth debate. Any theologically sound Christian believes that anyone who truly trusts in Christ's blood is eternally secure (even if they are misguided in some areas e.g age of the earth, end-times events, etc.)
The reason the young-earth timeline is critical to many creationists and why they defend it so vigorously is this: the interpretation of Genesis 1 fundamentally effects one's understanding and application of the Gospel.
The 6000 years is not actually derived from 2 Peter 3:8, which is often taken out of context to prove the days of creation were, in fact long ages (as you said, it is a simile. And it refers to the end of time, not the beginning). Rather, it comes from the genealogies found in Genesis 5.
If the earth's age was not important, God would not have placed any of this in the Bible. But the fact that He explicitly says literal days and gives the age of the earth through genealogies means it is important.
It is important because "days" as long ages means death before sin. If this is really true, then why was Man's fall such a big deal? Why would Christ come to save us from the consequences of something that had been happening for the millions or billions of years before Man in God's "very good" creation?
What would that say about God Himself?
Not saying you need to change your views, but I hope you see why this is such a big deal to some.